Be careful using the cooper tubing to directly run your AP water through, as some of the metals can leach into the water and harm your fish in some cases. For a small system such as yours I would think about digging a deep hole outside your greenhouse and burying a coil or so of Pex pipe, then either circulating your AP water or a portion of it through there, or preferably use clean water in a closed loop with a separate pump, connected to another coil in a sump with your AP water and use it like a heat exchanger.